memory: return error in barebox_add_memory_bank ...

When a memory bank is already registered, return an error code
instead of throwing a bug. This can happen if a board has registered
a memory bank and the same bank is then probed from the devicetree.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

register sdram as resources ...

Also, request the sdram regions used by the barebox binary,
bss, malloc space and stack.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

introduce generic memory bank handling ...

On arm we have the concept of memory banks which can
be registered and iterated over. This is useful for
other architectures aswell, so add some generic infrastructure
for this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

rename include/mem_malloc.h to include/memory.h ...

Which is a better name and also better to collect other
things.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>